# Einstein did not plagiarize Poincaré - by Olivier Darrigol

It is commonly believed that relativity theory was the
creation of one man only, Albert Einstein in most cases, and Henri
Poincaré in
a few cases.

*principle
of relativity*, as he first formulated in the last years of the
nineteenth
century. In other words, the outcome of any physical experiment should
not be
affected by a global, uniform translation of the experimental setup. In
1900,
Poincaré realized that at least in a first approximation
Lorentz's transformed
space, time, and field coordinates were the coordinates measured by
terrestrial
observers moving through the ether, if only the measurements were done
under
natural conventions. In particular, Lorentz's "local time" was the
time measured by moving observers who synchronized their clocks by
optical
means and ignored their motion with respect to the ether. Consequently,
the
formal invariance of the fundamental equations directly translated into
the
concrete invariance of phenomena ruled by these equations.

When, in 1904, Lorentz improved his transformations to get
an almost complete invariance of the fundamental equations,
Poincaré soon
corrected them to get perfect invariance. Again, he associated this
formal
symmetry with the invariance of phenomena in any inertial frame of
reference.
Yet he did not completely abandon the ether. In his view there still
was a
"true time" measured by observers bound to the ether; and the time
measured by moving observers was only an "apparent time." Of course,
this distinction could only be a conventional one, since the principle
of
relativity forbade any experimental determination of the ether frame.
Poincaré
nonetheless maintained it in order to avoid reforming our ancestral,
Newtonian
concept of time.

Poincaré published a summary of his results
in the June 5th,
1905, issue of the *Comptes rendus. *On
June 30th of the same year the *Annalen
der Physik* received Albert Einstein's
famous memoir "On the electrodynamics of moving bodies." From
the start Einstein assumed the principle of relativity and the light
principle
according to which the velocity in a given inertial frame is a
constant. He
redefined the concepts of space and time accordingly; he derived the
Lorentz
transformations; he proved the invariance of the Maxwell-Lorentz
equations
through them. His theory shared the same group of transformations as
Poincaré's, and the experimental predictions were exactly the
same But it differed from
Poincaré's in three
important ways: It did without the ether; It placed the space and time
measured
in different inertial systems all on the same footing; and it divorced
the
"kinematics" (the relations between space and time measurements in
various frames) from the field dynamics. Minkowski further increased
the
contrast between the two theories by defining a 4-dimensional spacetime
structure through the Lorentz group (in the spirit of Felix Klein's
Erlangen
program, according to which a geometry is defined through the group of
transformations).

*Science
and hypothesis*, Einstein certainly knew about Poincaré's
relativity
principle and about his general criticism of the concepts of space and
time. He
was plausibly aware of Poincaré's interpretation of Lorentz's
local time
(1900), and also of the Lorentz transformations in the form of 1904. He
is not
likely to have seen Poincaré's article in the *Compte
rendus *before writing his own. Whatever inspiration he may
have found in his ample readings, his approach differed from Lorentz's
and Poincaré's
at the profound level of concept formation.